That thread LoveToCamp pointed to has a great example -- I'm thinking about doing that, except perhaps in something like expanded metal to conserve weight. I've got two pieces of metal bar stock welded across the tongue there to hold up a small 5-lb propane tank I use for my outside camp stove and grill, so it could just mount on those.
Several years ago, I contacted Partner Steel in Idaho about fabricating a custom aluminum box that I could mount to the frame under the TM. They make very high quality stuff -- I have a camping stove and anodized aluminum griddle from them that are indestructible, and will undoubtedly be passed on to my kids, and if they are willing to let it go, to my grandkids someday. They're also really nice folks. I sent them some dimensions, and they quoted something along the lines of several hundred dollars with shipping (about $400 + shipping). Weight was also an issue, even with aluminum and even before considering everything that would undoubtedly be stuffed inside, considering most TM's are already very close to weight limits on the tongue and axle.
Something like this, made to custom size:
http://partnersteel.com/LG42.htm
Let us know if you end up doing something!
Dave